It's true they used stock footage of the Enterprise-C in one shot, but the footage of the Excalibur actually shot for the episode, namely the wide shot of the fleet, clearly shows her to be of the later variant type.

The Excalibur was definitely a variant. I mean they even changed the decals on the model for it.

The thing is, all the work on the Excalibur for "Redemption II" was "wasted", because we never saw any of those decals on screen. There's nothing to indicate that the variant vessel seen in the fleet scene would be the Excalibur, as opposed to some generic Ambassador. We learn the names and registries of some of the 23 ships that eventually reach the border, but not all of them... Only seventeen are listed on the graphic we see, and even of those, most are never seen on screen and given a canonical class identity.

The FASA Ambassador design was included because it was referenced in the dialogue of "Conspiracy," along with the Paine class frigate from the same book.

Yup, FASA did the standard unimaginative thing of deciding that any ship mentioned by name must be the class ship, hence the "Paine class". They never did a "Renegade class", though...